1. Field of the Invention
The present invention relates to the field of mass spectrometry, and more particularly to the field of automatic gain control in mass spectrometry to avoid saturation and/or space charge effects.
2. Discussion of the Related Art
Automatic gain control (AGC) is a method of controlling the number of ions in an ion trap to avoid saturation and/or space charge effects. Conventionally, AGC includes first injecting ions into the ion trap for some predetermined time using some gating optical element, typically in a pre-scan. A measurement of the resultant signal in the pre-scan is taken, and a calculation is then performed to determine what injection time (i.e. how long the gate is open) is needed to yield a specified “target” amount of signal, the target being the optimum signal which avoids saturation or space charge effects in the trap. The calculation often uses a simple linear relationship between ion signal and injection time. This technique, therefore, assumes that the ion beam is uniform both in composition and intensity with respect to time, within the timescale of all injection times. That is, that the delivery of all ions from the ion source is linearly dependent on the injection time. However, such current AGC schemes are not applicable to temporally non-uniform sources since the beam cannot be linearly sampled using a variable injection time.
Background information for a mass spectrometer apparatus that utilizes an automatic ion supply control feature, is described and claimed in U.S. Pat. No. 5,572,022, entitled “Method And Apparatus Of Increasing Dynamic Range And Sensitivity Of A Mass Spectrometer,” issued Nov. 5, 1996, to Schwartz et al., including the following, “An increased number of sample ions are introduced into the mass spectrometer for mass analysis with the aid of an automatic ion supply control, or feedback feature. The feedback portion of the invention controls the gating time, and hence the number of sample ions gated into the mass spectrometer, based on previous measurements of the ion content in the mass spectrometer to gate an amount relative to where space charge and saturation begins.”
Additional background information for a spectrometer AGC method and system configured for a temporally continuous beam instrument, is described and claimed in U.S. Pat. No. 7,323,682 B2, entitled “Pulsed Ion Source For Quadrupole Mass Spectrometer And Method,” issued Jan. 29, 2008, to McCauley et al., including the following, “In general, a variable duty cycle ion source assembly is coupled to a continuous beam mass spectrometer. The duty cycle can be adjusted based on a previous mass resolved scan data or real time sampling of ion intensities during mass analysis. This provides the ability to control the total number of ions formed and detected for any given mass during mass analysis. . . . In particular, this method allows ion abundance control on a mass-to-mass basis using a continuous beam device.”